More Problems - More Questions and AnswersFebruary 1967 QST Article

Wax nostalgic about and learn from the history of early electronics. See articles
from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

Here
is a good old fashion Q & A session on Ham radio topics... with
the emphasis on 'old.' QST magazine published a couple of these
columns in the 1960s, and this is the second in the series. I didn't
read anything that wouldn't be applicable today, especially if you
have some vintage gear. As with most such articles, there is something
to be learned by just about anyone who deals with electronics, especially
in the RF realm. One particularly interesting part is where the
author, in response to a question about building and tuning your
own radio, states, "Too many beginners are concerned about making
'Chinese Copies' of [manufactured] equipment described, even down
to the same placement of nuts and bolts." Little did he know then
that nearly every piece of equipment purchased new by Amateurs nowadays
would actually be made in China and not in the USA. ...
Heavy sigh(this link goes to a Mork &
Mindy video clip).

Beginner and Novice

More Problems - More Questions
and Answers

By Lewis G. McCoy, W1ICP

The answers to some of these questions may prove helpful to
you, both in time and trouble. It's useful information, so read
on.

Some time back, we had a question-and-answer article1
on problems that are common to many newcomers. Since then, a new
batch of questions has been collected, and the answers may be of
help to hams with similar problems.

Q. "I am using
an 18-foot vertical antenna, ground-mounted on my front lawn where
it is subject to being touched or contacted by children.

How much of a safety heard is this if a person touches it while
I am transmitting? What is the danger, if any, at 75-watts input
against what it might be at one kilowatt, and are some bands less
of a hazard than others?"

A.First and foremost, any antenna or feed line should be installed
so as to be protected from any physical contact. The antenna or
feed line can present a very serious hazard. There is always radio-frequency
energy present on an antenna when transmitting. While r.f. is rarely
present in a degree that could be considered lethal, even at a kilowatt
level, very severe r.f. burns can occur.

For the benefit
of the newcomer to amateur radio, r.f. voltages will not give you
an ordinary electrical shock but will cause a penetrating burn or
actual "cooking" of the flesh. Such a burn is slow to heal. The
greater the power the larger the amount of r.f. energy present on
the antenna. However, it is possible to get a nasty r.f. burn even
when running relatively low power, such as 50-watts input, and on
any band.

Still another hazard is that of getting an electrical
shock from the antenna. Fig. 1 shows the typical pi-network tank
circuit which is common these days. C1 is the blocking
capacitor which permits r.f. to flow from the plate of the tube
to the pi-network tank circuit and thence to the antenna. C1
prevents the plus-B voltage from being on the antenna circuit. If
C1 should short out - and it is not a rare occurrence
for capacitors to short - the plus-B voltage will flow out the feed
line to the antenna. Anyone coming in contact with the antenna could
be electrocuted.

One method of protecting against this is
to install an r.f. choke between the output lead and ground, as
suggested at RFC1 in Fig. 1. If the blocking capacitor
shorts, the plus-B will be shorted to ground via the choke and the
fuse in the transmitter power supply will blow.

Build a fence
around your antenna or put the antenna up in the air where no one
can touch it. If you are doing antenna work yourself, be sure the
power is completely off in the rig. If you have insurance, such
as the home-owner's type, be sure to read the fine print. Certain
restrictions on amateur stations are laid down by the National Electrical
Code and your insurance could be void if you don't comply with the
code. Your local library or building inspector's office will have
a copy of the wiring code so it would be a good idea to check. (Along
the same lines, although not pertinent to the question, if you have
a tower and think you have it insured, you had better check to make
sure. In many homeowners' policies a separate rider is required
for adequate coverage.)

Q. "None of
the articles on transmatches I have read in QST, the Handbook or
elsewhere, have ever mentioned shielding of such circuits. Photos
invariably show a chassis and a front panel but no signs of an enclosure
behind the panel. This is surprising since stress is usually placed
on shielding transmitters, especially around the output circuits.
Do transmatches require shielding or don't they?"

A.The answer is no, they don't. It is true that transmitters
require extensive shielding to prevent radiation of harmonics that
could cause TVI. Once the transmitter is shielded there is only
one way for harmonics to get out, and that is via the antenna terminal.
Any harmonics coming out of the transmitter should be attenuated
by use of a low-pass filter. In such a case, it will make no difference
if the transmatch is shielded. There are no harmonics reaching the
transmatch so there is no point in unnecessary shielding. If you
have harmonics that could cause TVI they must be kept enclosed within
the transmitter and suppressed with a filter.

On
the other hand, a transmatch is useful in suppressing the lower-frequency
harmonics - those that can cause problems by interfering with other
services than television. Because the transmatch is a selective
circuit tuned to your operating frequency, it will present a load
to the transmitter that is optimum for the operating frequency but
is not optimum for the harmonics. Therefore, harmonics will be attenuated
through the transmatch. But in either case, low-frequency or TVI
harmonics, the transmatch doesn't require shielding.

Q. "My
rig was working OK, as I was making contacts. Now, all of a sudden,
I can't work out at all. Can you tell me what is wrong or what to
check? The rig loads up into a light bulb OK and the antenna seems
to be the same."

A.There
are several possibilities in such a case. First and most important
is to make sure that you are transmitting and listening on the same
band. Believe it or not, many Novices think they are tuning up on
the right band when actually the output is on another. You can spend
a lot of frustrating time by calling on 40 while listening on 80!

One method of checking is to use a wavemeter to make sure that
you are on the right band. A recent QST article2 had
the construction details for a simple wavemeter and s.w.r./output-indicator
combination, which leads us to another point.

Without
such an indicator it is possible for a rig to be tuned up in what
appears a normal manner when, in fact, there is no power going to
the antenna. You could have an "open" or "short" in the feed line
and not know it, if you have no means of checking. The simple device
mentioned above contains a reflectometer which monitors both the
standing-wave ratio and the output. If something goes wrong in the
antenna or feeders, the s.w.r. should show a change from the normal
reading. On the other hand, if something happens in the transmitter,
this would show up as a change in the output, as indicated by the
meter in the device. We highly recommend the use of a combination
instrument that shows which band you are tuned to, the match in
the system, and the output, because by simple elimination you can
quickly pinpoint your troubles.

Q. "I recently
passed my amateur exam. I have been a CBer and I was wondering if
it is possible to use my CB beam on an amateur band. It is a 3-element
job with the elements in a vertical plane."

A.While this isn't a common question, the answer is worth passing
along to hams who may have an opportunity to pick up used (or new)
CB antennas at a bargain price.

The CB frequency assignments
are very close to the amateur 28-Mc. band, being centered around
27 Mc. As one goes higher in frequency, a half-wave antenna becomes
shorter. In this case, 27 Mc. is lower in frequency than the 28-Mc.
band so any CB antenna can be shortened to work on 28 Mc. The formula
for figuring the dimensions of a 28-Mc. beam is quite simple.

First decide on the portion of the band in which you plan to
do the most operating. Let's say it is around 28.6 Mc. Divide 28.6
in to 468 to give you the length in feet of the driven element.
The director should be made 5 percent shorter than the driven element
and the reflector 6 percent longer. The spacing between elements
does not need to be changed but it is a good idea to make the elements
horizontal to the earth. Most fixed stations on 28 Mc. use horizontally-polarized
antennas and you'll probably get better results if you mount your
antenna horizontally.

Q. "I would
like to build the Super-Duper 75-Watter described in...QST. Please
send me step-by-step wiring information and pictorial drawings of
the unit. I am afraid to attempt to build the rig with only the
schematic."

A.Variations of this request keep popping up. Unless specified
in an article, we have no pictorials, layout drawings, or step-by-step
information. If you want to do that kind of construction it is better
to buy a kit. Kits usually come complete with pictorials and step-by-step
instructions.

Here is the important point: A Novice must
acquire a certain amount of radio knowledge if he wants to stay
in amateur radio. In order to pass the FCC exams he must acquire
some "know-how" in radio circuitry. One of the best ways of acquiring
this knowledge is by building a piece of gear, trouble-shooting
it, and getting it working. Too many beginners are concerned about
making "Chinese Copies" of equipment described, even down to the
same placement of nuts and bolts. Don't be afraid to change the
layout. Don't be afraid to make substitutions. And above all, don't
be afraid to experiment. The worst that can happen is a burned-out
component or a blown fuse. The important thing is that you will
learn.

Q.
"I have an s.w.r. bridge and its dial is calibrated in watts along
with s.w.r. I am completely confused because I am reading 70 watts
output with 30 watts reflected, and I am only running 60 watts input.
What goes on...will the 30 watts coming back damage my rig?"

A.No, the 30 watts won't damage the rig. This is a complicated
thing to explain because a certain amount of knowledge is required
about reactance, phase, and transmission-line and antenna theory.
The subject is treated in detail in "The A.R.R.L. Antenna Book."

However, here's a simple analogy: If you go into a radio store
to buy a four-dollar capacitor and hand the clerk a ten-dollar bill,
you get six dollars back. The six dollars coming back doesn't hurt
your pocketbook. It just hasn't been spent. The same thing is true
of the reflected power in the above question. If you subtract the
30 watts from the 70 watts you'll find the remainder is 40 watts.
This is the total power the transmitter is putting out and is actually
going to the antenna to be radiated.

Still another way to
look at it is that with 60 watts input it would be pretty darn difficult
to get 100 watts output (70 forward, 30 reflected). The actual output
is the difference between the two, 40 watts.

Q. "I am building
the two-band receiver described in last month's QST and am having
a difficult time locating all the necessary parts. None of the local
radio stores seem to stock parts. Where do you suggest I look?"

A.Without a doubt this question (or a variation of it) is the
most common one. At one time, radio stores carried a fairly complete
line of parts, or would order them for you. It has become increasingly
difficult to obtain components, at least on a local basis. Not only
do the stores not stock components, but the manufacturers who make
the parts a ham would want have become less numerous.

If
you like building, you will almost certainly have to order parts
by mail. Although some distributors still stock a fairly wide range
of components, no one distributor is likely to have everything you
need. It is suggested you write to the large mail-order houses and
obtain their catalogs. Take a current issue of QST and go through
the index of advertisers and write those that have catalogs or flyers
available. You can, of course, write to those closest to you, but
if you want a complete "availability file" it is a good idea to
write to them all. Some of these concerns have two catalogs, a general
type and another larger one for industrial users. Also, nearly all
manufacturers of amateur equipment and components have catalogs
and usually, they are happy to send them on request. If you cannot
find a component in a distributor's catalog you can look it up in
the manufacturer's catalog and write and find out who sells the
item. In some cases, you'll find the manufacturer will sell direct.

We might add that at the ARRL, in any projects that are staff
constructed, every effort is made to use parts that are readily
available. However, even then it may be that a certain part is not
easily obtained. That's why it is a good idea to have a stock of
catalogs on hand. Also, as we stated in the previous question and
answer, don't be afraid to substitute components.

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
...

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